You can see Here the examples for adding, updating and retrieving Listitems using Web services.

XmlNode ndLists = Service.GetList("List"); //List name

XmlDocument doc = new XmlDocument();
doc.LoadXml(ndLists.OuterXml);

XmlNamespaceManager mg = new XmlNamespaceManager(doc.NameTable);
mg.AddNamespace("sp", "http://schemas.microsoft.com/sharepoint/soap/");
mg.AddNamespace("z", "#RowsetSchema");
mg.AddNamespace("rs", "urn:schemas-microsoft-com:rowset");
mg.AddNamespace("y", "http://schemas.microsoft.com/sharepoint/soap/ois");
mg.AddNamespace("w", "http://schemas.microsoft.com/WebPart/v2");
mg.AddNamespace("d", "http://schemas.microsoft.com/sharepoint/soap/directory");

XmlNodeList FieldsInList = doc.SelectNodes("//sp:Field", mg);

foreach (XmlNode Field in FieldsInList)
{
if (Field.HasChildNodes)
{
if (Field.Attributes["Name"].Value == "choiceFieldName") //choice fields internal name
{
foreach (XmlNode node in Field.ChildNodes)
{
if (node.HasChildNodes)
{
foreach (XmlNode Newnode in node.ChildNodes)
{
if(Newnode.HasChildNodes)
{
string value = Newnode.InnerText; // get node value to datatable
} } } } } } } }

Here are some video tutorials for customization of sharePoint site. ( its for beginners)